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The viscous coupling is a form of differential sometimes found in the transmission system of an all wheel drive vehicle. The viscous coupling links the drive shafts of the front and rear wheels of such a vehicle and allows torque to be transferred to one set of wheels if the other set starts spinning, (loses traction).

The coupling comprises two sets of plates inside a sealed housing which is filled with a viscous liquid. One set of plates is attached to the end of the front half of the drive shaft, whilst the other set connects to the rear half. The basic principle of operation is that the viscous fluid within the housing resists strong differences between the rotational speeds of the two sets of plates.

During normal driving conditions, both sets of plates and the liquid 'connecting' them within the coupling are all rotating at the same speed. However; If one set of wheels loses traction, its set of plates will rotate faster than the other set. In this case, the liquid within the viscous coupling transfers torque from the faster spinning plates to the slower spinning plates, ie torque is transferred to the set of wheels with more traction.

Viscous couplings are not restricted to all-wheel-drive applications. Viscous (or fluid) couplings are used extensively in the world of mechanical engineering to transfer motion from one place to another. The torque converter is probably one of the most commonly used devices utilizing viscous couplings.

In a viscous coupling, a fluid is pumped using the powerplant. The pressurized fluid is then directed to a drive shaft. The pressure in the fluid causes the drive shaft to move, thus creating a coupling between the two.

Advantages of viscous couplings:

  • Fewer moving parts - less maintanance and higher reliability
  • Speed of powerplant is not directly tied to the conditions of the drive shaft (e.g. the powerplant can continue to run without stalling even if the drivetrain has stopped.)
  • Has the potential to temporarily increase output torque
  • Effective gear ratios can be easily changed by varying the nozzle diameter - Bernoulli's Principle
  • Disadvantages:

  • There is always some slip between the two shafts, resulting in reduced efficiency
  • Cannot be used for as many applications as gears can
  • It is more difficult to design and manufacture
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